Austenitic castable high temperature alloy

ABSTRACT

An austenitic castable high temperature alloy having improved high temperature strength coupled with good corrosion resistance and ductility in the as-cast condition and comprising about 16 to about 22% chromium; about 6 to about 18% nickel; about 6 to about 10% molybdenum, up to about 3% tungsten; about 0.5 to about 2.5% boron; about 0.01 to about 0.4% carbon; about 0.15 to about 7% manganese; up to about 3% silicon; from zero to about 1% niobium; and the balance substantially all iron together with normal residuals and incidental impurities present in usual amounts.

United States Patent Sponseller 1 Sept. 2, 1975 [5 1 AUSTENITIC CASTABLEHIGH 3.352.666 11/1967 Foster Ct al. 75/1221 F x TEMPERATURE ALLOY IPrimurv ExaminerL. Dewayne Rutledge 7 I D d L. S A A b 5] nvemor 23ponsener nn r or Assistant bxu'mznerArthur J. Stemer Attorney. Agent, orFirmHarness, Dickey & Pierce [73] Assignee: AMAX Inc., New York, N.Y.

[22] Filed: May 13, 1974 [57] ABSTRACT Appl. No.: 469,347

US. Cl. 75/128 A; 75/128 C; 75/128 F;

75/128 G; 75/128 W Int. Cl. C22c 39/26; C220 39/50 Field of Search75/128 F. 128 W, 128 A,

An austenitic castable high temperature alloy having improved hightemperature strength coupled with good corrosion resistance andductility in the as-cast condition and comprising about 16 to about 22%chromium; about 6 to about 18% nickel; about 6 to about 10% molybdenum,up to about 3% tungsten; about 0.5 to-about 2.5% boron; about 0.01 toabout 0.4% carbon; about 0.15 to about 7% manganese; up to about 3%silicon; from zero to about 1% niobium; and the balance substantiallyall iron together with normal residuals and incidental impuritiespresent in usual amounts.-

3 Claims, N0 Drawings AUSTENITIC CASTABLF. HIGH TEMPERATURE ALLOYBACKGROUND OF THE INVENTION There has been an increasing need for acomparatively low-cost alloy possessed of good mechanical properties andcorrosion resistance at elevated temperatures to enable its use for thefabrication of cast components such blades. vanes. and integral wheelsfor gas turbine engines. and exhaust valves for internal combustionengines. This need has been accentuated by the potential widespread useof moderate-size gas turbine engines in a variety of consumer productsincluding trucks and automobiles. garden tractors. as well assnowmobiles. boats and miscellaneous recre ational vehicles. While avariety of so-called superalloys of the nickel. cobalt or iron-base aresuitable for use due to their excellent high temperature properties. therelatively high cost of such materials and the difficulty of fabricatingthem into components has detracted from their widespread use in themanufacture of competitively-priced consumer products.

In order to overcome the cost disadvantages associated with superalloymaterials. various iron-base alloys. particularly austenitic-type alloyscontaining chromium. have heretofore been proposed for use in thefabrication of components which are to be exposed to high temperatureand high stress environments during service. While some austenitic hightemperature alloys have provided a satisfactory low-cost substitute incer tain situations. the inferior mechanical properties at hightemperatures. and/or a general lack of ductility of such alloys hasdetracted from a more widespread adoption thereof. In most instances.such prior high temperature alloys have required relatively complexmelting and casting procedures and costly post-heat treatments to attainadequate properties which also have detracted from a greater acceptanceof such materials.

The present invention overcomes many of the problems and disadvantagesassociated with castable high temperature alloys of similar typeheretofore known by providing an iron-base austenitic alloy which canreadily be air-melted and. because of very high fluidity. can be cast inaccordance with standard foundry practices and wherein the resultantcastings are possessed of excellent properties in the as-cast condition.avoiding further costly and time-comsuming heat treating practices ofthe types heretofore employed. In addition to the excellent hightemperature stress rupture and corrosion resistant properties, thecomparatively low cost of the present alloy renders it eminentlysuitable for the manufacture of blades. vanes. and integral wheels forgas turbine engines. and poppet-type cxhaust valves for internalcombustion engines, which are exposed to corrosive environments attemperatures as high as about 1500F during service. Significant costsavings without any sacrifice in performance are obtainable bysubstituting the present alloy for nickelbase or cobalt-base superalloysfor the fabrication of components which are intended for use underservice conditions of moderate severity in which the use of such highcost superalloys ordinarily cannot be fully justified.

SUMMARY OF THE INVENTION The benefits and adyantages of the presentinvention are achieved by an iron-base austenitic alloy which iscomprised of a carefully selected group of alloying constituentsemployed in controlled amounts to provide a material having unique andbalanced properties. cnabling the alloy to be air-melted and cast inaccordance with standard foundry practices and wherein the resultanteast components are possessed of excellent room temperature and hightemperature mechanical properties and corrosion resistance in an as-castcondition. dispensing with the need of costly and time-consumingpost-heat treatments. The alloy is further characterized as one having astable microstructure which retains the improved properties of the alloyeven after prolonged exposure to elevated temperatures during service.

The castable austenitic alloy of the present invention contains about16% to about 22% chromium. about 6% to about 18%- nickel. and 6% toabout 1071 molybdenum. up to about 3% tungsten. 0.5% to about 2.571boron. about 0.01.71 to about 0.471 carbon. about 0.15% to about 7%manganese. up to about 3% silicon. from zero to about 1% niobium. andthe balance substantially all iron together with normal residuals andincidental impurities present in conventional amounts. The alloy isfurther characterized as one having a substantially austenitiemicrostructure in which borides and carbides are present in theinterdendritic and intergra n ular boundary phase networks. as well asfinely dispersed borides and carbides within the grains themselves. Theforegoing microstructure. together with the unique combination of thealloying elements employed, provide for the substantially improved hightempera ture stress rupture properties and mechanical strength of thealloy. as well as its substantially increased ductility. rendering iteminently suitable for the simple and economical fabrication of avariety of components and parts to be used in elevated temperatureservice conditions.

Additional benefits and advantages of the present invention will becomeapparent upon the reading of the description of the preferredembodiments and the specific examples provided.

DESCRIPTION OF THE PREFERRED EMBODIMENTS TABLE 1 Alloy CompositionPercent by Weight I Nominal Constituent Permissible Preferred Chromiuml6 2: l7 20 ll Nickel 6- l8 8 l4 l2 Molybdenum 6 10 7 9 8 Tungsten 3' Uh 3,

Boron 0.5 2.5 (Lo 1.3 1.0

TABLE l-Continued Alloy Composition Percent by Weight It will beunderstood that in addition to the specific alloying elements enumeratedin Table 1, the alloy of the present invention can also containconventional residuals and normal impurities present in the amountsnormally encountered in commercial steel-making practices. Suchresiduals and impurities, when present in normal quantities, do notadversely affect the properties of the alloy which provides forincreased flexibility and efficiency in the utilization of scrap iron inaccordance with commercial foundry practices.

The chromium constituent as set forth in the table can be employed inamounts ranging from 16 up to about 22%. while quantities in the rangeof about 17 to 20% are usually preferred. The chromium constituentcontributes oxidation or corrosion resistance to the alloy and alsocomprises a boride former to produce precipitated chromium boride in theinterdendritic and intergranular boundary phase network within theaustenitic microstrueture of the alloy. Quantities of chromium less thanabout 16% result in inadequate corrosion resistance of the alloy, whilequantitiesin excess of about 22% result in instability of themicrostructure and the formation of undesirable phases during elevatedtemperature service of components'east of the alloy.

Nickel can be employed in amounts broadly ranging from about 6 up toabout 18% and contributes to the formation and stability of theaustenitic structure of the alloy. Quantities of nickel less than about6% result in the formation of an undesirable proportion of ferrite,while quantities in excess of about 18% provide'no appreciable benefitsover that obtained by the use of lesser amounts and. therefore, the useof amounts greater than about 18% is undesirable from an economicstandpoint.

Molybdenum is employed in an amount ranging from about 6 to about 10%,and preferably from about 7 to about 9%. The molybdenum constituentcontributes strength to the austenitic alloy. both through solidsolutionstrengthening and by the formation of the intergranular andinterdendritic boride and carbide boundary phases. Quantities ofmolybdenum less than about 6% result in alloys which are generally ofinadequate high temperature strength; while quantities in excess ofabout 10%. depending upon the quantities of other alloying agentspresent. tend to impart instability to the microstructure of the alloyand the formation of certain undesirable phases during service The useof tungsten in the alloy is optional and quantities up to about 3% canbe employed for enhancing the strength of the alloy by bothsolid-solution strengthening and the formation of carbide and boridephases in the boundary network. Quantities of tungsten greater thanabout 3% are undesirable due to the in creased instability of themicrostructure and the formation of undesirable phases during service ofcast componentsat elevated temperatures. As will be noted in Table l aparticularly satisfactory alloy of the enumerated nominal compositionneed not contain any tungsten in order to achieve the excellent hightemperature stress rupture properties coupled with comparativelysuperior roomternperature ductility.

The boron alloying constituent contributes to the interdendritic andintergranular boride phase strengthening mechanism by the formation ofborides containing chromium and molybdenum, as well as tungsten whenpresent. A fine dispersion of borides within the austenite grains alsocontributes toward the improved strength properties of the alloy atelevated temperatures. Generally. quantities of boron less than about0.5% result in an alloy of inadequate strength due to the insufficientformation of the interdendritic and intergranular boride phase network,whereas quantities above about 2.5% generally result in an excessiveembrittlement and an undesirable loss of ductility of the as-castmaterial. It is for this reason that the element boron is controlledwithin the specific ranges as set forth in Table l.

The presence of carbon contributes to the elevatedtemperature strengthof the alloy by the formation of intergranular phases consisting ofprecipitated carbides of niobium, molybdenum and tungsten, if present.The carbon also contributes to improved mechanical properties by theformation of finely-dispersed carbide phases, principally niobiumcarbide, in the austenite. As set forth in Table l, the carbon contentcan range from as low as about 0.01 to as high as 0.4%, while quantitiesof from about 0.05 to about 0.25% are preferred. The carbon constituentis controlled within the aforementioncd range since quantities in excessof about 0.4% result in excessive brittleness and a loss in theductility of the alloy.

The use of manganese may broadly range from as low as about 0.15 to ashigh as about 7%, although quantities of about 0.5 to about 5% arepreferred. As in the case of nickel. the managanese contributes towardthe stability of the austenitic structure of the alloy and the specificconcentration employed in the alloy can be varied in consideration ofthe quantity of nickel employed. Normally. quantities of manganese inexcess of about 7% are objectionable because of the high degree ofreactivity of the molten alloy with the melting vessel, as well as thematerial of which the casting mold is comprised. When the alloy is castinto molds that have been preheated to high temperatures, such asapproximately 18()0F, it is desirable to employ a manganese content of3% or less.

Silicon comprises an optional constituent in the alloy and generally canbe tolerated in amounts up to about 3%, while concentrations of about0.15%. which corresponds to a normal residual level. up to about 1% arepreferred. When silicon is present in amounts generally greater thanabout 3%. the presence of such excessive quantities promotes theformation of ferrite and, accordingly. concentrations of this magnitudeand above are objectionable] Niobium also comprises an optionalingredient. although its presence in concentrations of about 0.2 toabout 0.7% is preferred. The inclusion of niobium in the alloy resultsin an enhancement of the high temperature strength properties of thealloy as a result of the formation offinely-dispersed niobium carbidephase in the austenite. as well as in the interdendritic andintergranular phase network. The use of niobium in amounts generallygreater than about it: is undesirable due to the resultant reduction inductility of the alloy.

The balance of the alloy consists essentially-of iron while the nickelyvas added as electrolytic nickel and from that present in ferroalloys.was added as eommercially pure iron. The melts were deoxidi7ed withalumi along with conventional residuals and incidental impu- 5 num and\\'er. e poured into test bar specimens using rities present in theusual quantities. A further benefitpreheated lost wax-type molds. Thespecific composiof the alloy of the present invention is in its apparenttion of the experimental alloys is set forth in Table 2. toleration oftrace quantities of such elements as lead. tin. arsenic. antimony,copper. sulfur. phosphorus. etc.. i with any significant detrimentaleffects on its properl H TABLE 2 ties, 1

Commercial-sized heats of the alloy can be prepared Composition ofExperimental Alloys utilizing standard Mr-melting practices and theboron v v Pcrwnhy weight constituent is preferably added ust pr or topouring to 51cm! n ,7 All") 3 AH). C avoid appreciable oxidationthereof. The alloy is castad. l 1 v ble in accordance with standardcasting practlce. and mm. the cast components. after suitable finishingand ma- Molybdenum I 0 7.5 v n chining as may be required. can beemployed directly 3 3 without need for further'heat treatment. It willbe apparent from the foregoing thatthe comparatively low 2 2%:- t 1 0.0.cost of the alloycoupled with the ease 1n casting comjf jj (H5 (H5 I 3ponents therefrom which do not require further heat Silicon 0.25 0.250.25 treatment provides for significant cost savings over cas- Mimetable austenitic steel alloys of the types heretofore 7 known. T Y 1 MIn order to further illustrate the austenitic alloy of the The test barspecimens obtained of each experimenpresent invention,the followingexamples are provided. tal alloy were subjected to physical andmechanical It will be understo d that th s exa p r ppl testing,including the determination of their respective merely for illustrativepurposes and are not intended to w room t m erature-- density, theirroom temperature be limiting of the scope of the invention as herein de7 Y 01% offs t i ld Strength (Y5), d l i il scribed and as set forth inthe subjoined claims. strength (U l S). their. elevated-temperatureultimate EXAMPLE 1 tensile strength (UTS), as wellas their percentelonga-1 tion at room temperature and elevated temperatures as Threeexperimental alloys. designated A. B d 15 an indication of the ductilityof the sample alloys. The respectively. were prepared by air-melting55-p nd alloys were also subjected to high temp'erature'stressheats inan induction furnace. Th Chr m um. molybrupture tests and the IOU-hourrupture'strength was dedenum. tungsten, niobium, boron, manganese andsilitermined. The data obtained are summarized in Table con alloyingelements were introduced as ferroalloys; 3.

TABLE 3 Properties of Experimental Alloys 100 Hr. Rupture Temp. DensityTensile Properties Strength Alloy F (lb/in) YS (ksi) UTS (ksi)Elong.('71 (ksi) 800 94.2 3.0 1000 89.0 3.0 1200 86.9 2.5 as 1300 83.33.5 52 1350 44 1400 77.1 3.5 37 1500 (16.3 6.0 I600 49.3 7.0 17

1200 91.2 4.5 an 1300 241.9 4.0 1350 40 1400 m0 5.0 as 1500 6L0 0,0 25I600 4&4 1x0 17 Elongation at 75W determined at the moment of fracture hresistance strain gauges.

It will be apparent from a review of the' data as set forth in Table 3.that thecreep-rupture properties of ity of each of the experimentalalloys as indicated by the percent elongation thereof is unexpectedlyhigh in comparison to the ductility of steel alloys of similar typeheretofore known. This is a consequence of the relatively high boron andrelatively low carbon contents of these alloys. The experimental alloysare also observed to possess good high temperature corrosion resistance.With respect to the density of the alloys. it will be noted that thevalues obtained for the alloys of the present invention aresignificantly lower than densi ties of cobalt-base superalloys and someof the more recently developed nickel-base superalloys. which aresuitable for use under similar service conditions, thereby providing fora significant reduction in the stress of rotating gas turbinecomponents.

EXAMPLE 2 An experimental alloy. designated as D, is prepared byair-melting a heat containing the alloying elements asset forth in Table1 'under the heading Nominal Composition." From the results obtained onthe experimentalvalloys of Example 1. the anticipated properties ofalloy- D are a density of 0.287 pounds per cubic inch;

a 0.271 offset yield strength of .65 ksi, an ultimate tensile strengthof 105 ksi. an elongation of -l.257 all at room temperature. and arupture strength at l.350F of 48 ksi at 100 hours. The averagecocfficient of thermal expansion of the alloy is anticipated to be 9.2 Xl0 inch per inch per F. over the range of 70F to 1,350F.

'While it will be apparent that the invention herein disclosed iswellcalculated to achieve the benefits and advantages herein set forth, itwill be appreciated that the invention is susceptible to modification,variation and change without departing from the'spirit thereof. What isclaimed is: v l. Anau'stenitic castable high temperature alloyconsisting essentially of about 16 to about 22% chromium, about 6 toabout l 87( nickel, about'6 to about 10% molybde num. up to about 371tungsten, and about 0.5 to about 2.571 boron, about 0.01 to about0.47rcarbon, about 0. l 5 to about 7% manganese, up to about 3% silicon.from zero to about 1% niobium, and the balance substantially all irontogether with normal residuals and incidental impurities'present inusual amounts, said alloy further characterizedas having a IOO-hourrupture strength at l,500F of at least about 25 ksi;

2. The austenitic castable high temperature alloy as defined in claim 1,in whichjch'romium'is present in an amount of about 17 to about 20%,nickel'is present in an amount of about 8 to about 14 /1. molybdenum ispresent in an amount of about 7 to abou t97z boron is present in anamount of about 0.6 to about l .370, carbon is present in an amount ofabout 0.05 to about 0.25%. manganese-is present in an amount of about0.5 to about 5%. silicon is present in anamo unt' of about 0.15 to aboutl /r. and niobium is present in an amount of about 0.2 to about 0.7%. ii

3. The austenitic castable high temp er' ture alloy as defined in claim1, in which chtorniu'rr'i is'p'resent in an amountof about 18%, nickelis present in an amo'unt

1. AN AUSTENITIC CASTABLE HIGH TEMPERATURE ALLOY CONSISTING ESSENTIALLYOF ABOUT 16 TO ABOUT 22% CHROIUM, ABOUT 6 TO ABOUT 18% NICKEL, ABOUT 6TO ABOUT 10% MOLYBDENUM, UP TO ABOUT 3% TUNGSTEN, AND ABOUT 0.5 TO ABOUT2.5% BORON, ABOUT 0.01 TO ABOUT 0.4% CARBON, ABOUT 0.15 TO ABOUT 7%MAGANESE, UP TO ABOUT 3% SILICON, FROM ZERO TO ABOUT 1% NIOBIUM AND THEBALANCE SUBSTANTIALLY ALL IRON TOGEHTER WITH NORMAL RESIDUALS ANDINCIDENTAL IMPURITIES PRESENT IN USUAL AMOUNTS, SAID ALLOY FURTHERCHARACTERIZED AS HAVING A 100-HOUR RUPTURE STRENGTH AT 1,500*F OF ATLEAST 25 KSI.
 2. The austenitic castable high temperature alloy asdefined in claim 1, in which chromium is present in an amount of about17 to about 20%, nickel is present in an amount of about 8 to about 14%,molybdenum is present in an amount of about 7 to about 9%, boron ispresent in an amount of about 0.6 to about 1.3%, carbon is present in anamount of about 0.05 to about 0.25%, manganese is present in an amountof about 0.5 to about 5%, silicon is present in an amount of about 0.15to about 1%, and niobium is present in an amount of about 0.2 to about0.7%.
 3. The austenitic castable high temperature alloy as defined inclaim 1, in which chromium is present in an amount of about 18%, nickelis present in an amount of about 12%, molybdenum is present in an amountof about 8%, boron is present in an amount of about 1%, carbon ispresent in an amount of about 0.2%, manganese is present in an amount ofabout 1%, silicon is present in an amount of about 0.5% and niobium ispresent in an amount of about 0.5%.